Thank you acrylian for your answer.

BO > Back Office, admin
Sorry for the abbreviation...

For further multi language support, is it possible have a feature like "synonym" on locale setting? that is to define some kind of browser language equal to a locale installed if "http accept language" chosen.

I have not sure is there other language have similar case, but for chinese, there is got zh_HK, zh_TW, zh_CN, zh_sg. They may have different wording/written form, but basically the people could understand each other wording.

So that if only have zh_TW installed, I can set something like (zh_HK|zh_CN|zh_sg) equal to zh_TW, zenphoto will output zh_TW translation for those 3 defined in browser.

There must be a 1 to 1 correspondence between the language specifier and a folder in the locales folders. Your best bet is to use zh_CN which will show simply as Chinese. Alternatively you can replicate the folders.

Other languages, specially Spanish have a similar situation. ez_ES displays simply as Spanish.

BTW, our tables do not include zh_sg, should it be added?

You should also check to see what the HTTP Accept Language returns. It may already have the list zh_TW|zh_HK|zh_CN|zh_sg. If it returns the list zenphoto will try to match each in the list and will take the first match.